Master clock system



1961 G. P. s'rou'r 2,994,184

MASTER CLOCK SYSTEM Filed Sept. 16, 1955 2 Sheets-Sheet l INVENTOR GEORGE PHILJP STOUT 94 ATT RNEYS Aug. 1, 1961 G. P. STOUT MASTER CLOCK SYSTEM 2 Sheets-Sheet 2 Filed Sept. 16, 1955 I I ,1} MAX. RATELCRITICAL) mOdFJO 432E021 OVER FREQUEN VOLTAGE TIME INVENTOR GEORGE PHILIP STOUT XITORNEYS MM m 1 United States Patent This invention relates to electric clock systems and more particularly to an electric clock system including an auxiliary standby power source used in the event of failure of the normal power supply.

The clock system disclosed herein is particularly intended for use in small industrial plants, schools and institutions. 'At present, two types of clock systems are in general use in installations of the type just mentioned. One of these systems employs a plurality of conventional synchronous-type clocks which are installed throughout the premises and plugged into the most convenient outlet. This .system has the objection that it is very difficult to synchronize the plurality of clocks and, further, that the clocks are disabled by power failure either in a particular area or for the entire plant or institution.

The other system which is in current use in an independent clock system utilizing a master clock which in turn controls a multiplicity of secondary clocks throughout the premises. The secondary clocks are synchronized with the master clock by a series of pulses which are generated in the master clock, rather than being synchronized with the frequency of the local power supply. In some cases, the system is wholly dependent on the local power supply, although the secondary clocks are not synchronously driven from that supply. This independent clock system. has the disadvantage that it is quite expensive, which excludes the smaller user, and, furthermore, the degree of accuracy is not as good as that maintained by the synchronous clocks of the firstmentioned system which depend on the frequency of the power system. The master clocks of independent clock systems usually are of the pendulum type which accumulate a positive or negative error and have a degree of instability due to factors inherent in pendulum clocks.

Accordingly, it is an object of this invention to provide an electric clock system particularly suitable for use in industrial plants, schools, and other institutions, which combines the synchronization advantages of an independent clock systemv controlled by a master clock with the accuracy inherent in synchronously driven clocks which operate in synchronism with the local power supply.

It is another object of this invention to provide a lowvoltage electric clock system which permits electrical power to be supplied to the clocks through low-voltage conductors such as twisted telephone wires.

Itis still another object of this invention to provide a master clock system which is normally operated in synchronism with the frequency of the local alternating current power supply but having a standby source of alternating current power which automatically becomes elfective in the event of the failure of the normal source of power.

It is a further object of this invention to provide a master clock system having a plurality ofauxiliary alteri nating current'power supplies of different frequencies with means for selecting the auxiliary power supply to be connected to the clock system.

It is a still further object of this invention to provide a master clock system including a plurality of clocks normally driven in synchronism with the local alternating-i current power supplyand also including means for easily correcting for positive ,and negative time indication errors of the clocks. V q

1 I i s l mbe e t Of hi invention to P ov e n Patented Aug. 1, 1961 apparatus and method for applying electric power of a much-higher-than-normal frequency to a synchronous. electric clock system to correct negative time errors of the system.

In achievement of these objectives, there is provided in accordance with this invention a master clock system including a plurality of synchronous clocks which are normally connected through a step-down transformer to the local power supply and operate in synchronism therewith. An auxiliary power supply is provided for use in the event of failure of the normal alternating-current power supply. This auxiliary supply may include a storage battery, which is charged from the normal alternatingcurrent power supply, to drive an inverter having an alternating-current output of the same frequency as the frequency of the normal power source. A relay responsive to voltage failure of the normal power supply for the clocks connects the alternating-current output of the auxiliary supply to the clocks.

In order to correct negative time indication errors of the clocks, a second auxiliary alternating-current generator is provided having an alternating-current output of a higher frequency than the frequency of the normal power supply. This second auxiliary alternating-current generator may be connected to the clocks through a selector switch for correction of negative errors of the clocks independently of the relay which responds to voltage failure. The selector switch may also be turned to a position in which the clocks are disconnected from all sources of power, including the normal and auxiliary power sources, to permit correction of positive errors in the time indication of the clocks. Further features of the system include a time counter which logs the duration of the power outage of the normal power supply, as well as an indicator light which indicates that a failure of the normal power supply has occurred upon the resump: tion of the normal power supply.

A modified apparatus and method are also provided for correcting negative time indication errors of the clock system by connecting an auxiliary power source to the clock system having a much higher frequency than the normal frequency of the clock system. The frequency of the auxiliary power source is increased at a rate which does not exceed a predetermined critical rate, dependent on the ability of the clock system to maintain synchronism, until the auxiliary power source has reached a predetermined frequency and voltage much higher than the normal frequency and voltage of the clock system. This predetermined frequency and voltage are then continuously applied until the negative error of the clock system has been corrected.

Further features and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is an electrical circuit diagram of a master clock system in accordance with the invention;

FIG. 2 is a schematic diagram of an apparatus which may be used to provide electric power of much-higherthan-normal frequency to the clock system to correct negative time indications of the clock system;

FIG. 3 is a graphical analysis of the volt-age and frequency relationships when applying high frequency power to the clock system; and

FIG. 4 is a schematic diagram of the electrical and mechanical components of the apparatus of FIG. 2.

Referring now to FIG. 1, there are shown a plurality of synchronous electric clocks generally indicated at 10 and a monitor clock 12, each connected across couductors 14 and 16 which are connected to the secondary winding 20 of a step-down transformer generally indicated at 18. Monitor clock- 12. is a synchronous clock similar to clocks 10 but is installed on the switchboardor control cabinet of the apparatus. Clocks 10 and 12 are each provided with a self-starting synchronous alternating motor of the type used in conventional synchronous electric clocks. Each clock includes an alternating-current stator winding adapted to be energized from a 24'-volt alternating-current power supply. Each clock is also provided with a removable reset key which permits the hands of each clock to be individually adjusted. Transformer 18 includes a primary winding 22 to the opposite terminals 23 and. 25 of which are connected conductors 24 and 26. Transformer winding 22 is nonnally connected to power lines 28 and 30 which are across the normal source of alternating-current power for clocks 10 and 12, assumed to be a 115-volt, 60-cycle supply. The connection from normal power supply lines 28 and 30 to transformer primary winding 22 is effected through a relay generally indicated at 32 and a selector switch generally indicated at 34. The ll-volt input to primary Winding 22 is stepped down to 24 volts by secondary winding 20. Thus, clocks and 12 are connected to a low voltage supply, permitting the use of twisted telephone wire for the conductors 1'4 and 16 across. which the clocks are connected.

Relay 32 includes an operating coil 36 which controls a. pair of movable contacts 38 and 44. One side of operating coil 36 is connected by conductorv 37 directly to power line 28. The other side of operating coil 36 is connected by conductor 39, junction 41, conductor 60 and junction 62 to the other power line 30. Operating coil 36 is thereby directly responsive to the energization condition of power lines 28 and 30. Contact 38 is actuated by operating coil 36 to engage fixed contact 40 when the voltage across normal power supply lines 28 and 30 is sufiicient to operate clocks 10 and- 12. When coil 36 is not energized, or is insufficiently energized, movable contact 38 engages fixed contact 42. Under the same conditions in which contact 38 engages fixed contact 40; namely, proper energization of power lines 28 and 30, movable contact 44 is actuated by coil 36 to an open circuit condition. However, when coil 36 is not energized, or is insufficiently energized, movable contact 44 engages fixed contact 46.

Selector switch 34 includes a movable contact 56 which is connected to conductor 26 and thus to terminal 23 of transformer primary winding 22. Movable contact 56 is selectively engageable with fixed contacts 50, 52 and 54. In. the normal position of selector switch 34, movable contact 56 engages fixed contact 50, and thus isconnected through conductor '48 to either fixed contact 40 or 42 of" relay 32.

When power lines 28 and 30 are properly energized from the normal source of alternating-current power so that relay coil 36 is energized, and with selector switch 34 in the normal position in which movable contact 56 engages fixed contact 50, transformer primary winding 22 is connected across power lines 28 and 30 through the following circuit: trom power line 28 to fixed contact 40 of relay 32, through movable contact 38 of relay 32, through conductor 48 to fixed contact 50 of selector switch 34, through movable contact 56 of the selector switch to conductor 26, and through conductor 26' to terminal 23 of winding 22. The other terminal 25 of primary winding 22 is directly connected to power line 30 through conductor 24, junction 58, conductor 60, and junction 62.

A second selector switch generally indicated at 64 is operated in tandem with selector switch 34 for a purpose to be hereinafter described. Selector switch 64 includes three fixed contacts 66, 68 and 70, of which contacts 68 and 70 are blanks or open-circuit positions, and a movable contact 72 which selectively engages fixed. contacts 66', 68 and 70.

Auxiliary power supply Inorder to operate clocks 10 and 12 in the event of a fiailure of the normal power supply, or if the voltage of the normal power supply should become too low to operate the clocks, an auxiliary power supply is provided which now be described.

The auxiliary power supply includes a storage battery 74 having terminals 75 and 77 which are connected to the output of a rectifier bridge 76. Rectifier bridge 76 derives its input power from secondary winding 82 of a step-down transformer generally indicated at 80. The primary 84 of transformer 80 is connected through an adjustable resistance 86 across power lines 28 and 30 of the normal alternating-current power supply. Battery 7 4 is trickle-charged through rectifier 76 when power lines 28 and 30 are energized, battery 74 being charged to a voltage of 24 volts, for example.

An auxiliary alternatingmurrent generator in the form of an inverter '88 is provided. Inverter 88 is adapted to receive a 24-'volt direct-current input from battery 74 at the input terminals 90 and 92 of the inverter, and v is adapted to deliver a l15-volt, 60-cycle alternating-current output at its output terminals 94 and '96. Inverter 88 may be of the vibrator type frequently used inradio power supplies. The direct-current input terminal 90 of inverter 88 is connected by oonductor98 to fixed con.- tact 46 ofrelay 32. The other input terminal 92 of the inverter is connected by conductor 100, junction 102 and conductor 104 to terminal 77 of battery 74. Movable contact 44 of relay 32 is connected through conductor 106 and junction 108 to conductor 110 which is con nected to terminal 75 of battery 74.

When relay coil 36 of relay 32 is de-energized due to. a failure of the normal power supply across power lines 28 and 30, or is insufliciently energized, terminal 90 of the winding 22 of transformer 18. The other alternatingcurrent output terminal 94 of inverter 88- is connected through conductor 116 to fixed contact 42 of relay 32. When relay 32 is de-energized due to failure of the normal power supply across lines 28 and 30, movable contact 38 of the relay is in the position shown in the drawing in which it engages fixed contact 42. In this position, output terminal 94 of the inverter is connected through con; ductor 116, fixed contact 42, movable contact 38, conductor 48, and fixed and movable contacts 50 and of selector switch 34 through conductor 26 to terminal 23 of primary winding 22. Thus, in the position ofthe movable relay contacts 42 and 44, shown in the drawing, corresponding to the de-energized condition of relay 32 due to a power outage on lines 28 and 30, the directcurrent input terminals and 92 of inverter 88 are con nected to battery 74 while the alternating-current output terminals 94 and 96 of the inverter are connected to primary winding 22 of transformer 18. In this condition, inverter 88' is supplying a -volt, 60-cycle power supply to clocks 10 and 12 through transformer 18.

An indicating light 117 is connected across power lines 28' and 30 of the normal power supply to indicate when'these lines are energized. As a further feature of the-system, a pilot light 118 is provided to indicate when the normal alternating-current power supply across lines 28 and 30 has failed. Pilot light 118 operates in conjunction with a relay generally indicated at 120 having an operating coil 122 andamovablecontact 124. Operatingcoil 122 is connected across power line 28-by means: of conductor 126. The-other side of operating coil 122is connected by a conductor128 to fixed contact 130 of the relay. Fixed contact 130 is also connected by means'of a conductor 132 in series witha push button 135 to conductor 60 and thus to power line 30. One side of indicating light 118 is connected by conductors 134 and 126 to power line 28. The other side of the indicating light is connected by means of conductor 136 to fixed contact 138 'of relay 120. When power lines 28 and 30 are energized by the normal source of alternating-current power, operating coil 122 is energized through conductors 126, 128, 132 and 60 when push button 135 is momentarily closed. Energization of relay operating coil 122 causes movable contact 124 of the relay to engage fixed contact 130 thereby closing a holding circuit for coil 122 through conductors 137 and 60 to power line 30. At the same time, movement of movable contact 124 out of engagement with fixed contact 138 opens the energization circuit of indicating light 118. As long as power lines 28 and 30 remain energized by the normal source of alternating-current power, operating coil 122 remains energized and maintains movable contact 124 in engagement with fixed contact 130 and out of engagement with fixed contact 138. Hence, indicating light 118 remains de-energized as long as lines 28 and 30 are energized from the normal source of alternating-current power. However, if lines 28 and 30 should become de-energized due to a failure of the normal alternating-current power source, coil 122 becomes d e-energized and causes contact 124 to move out of engagement with fixed contact 130 and into engagement with fixed contact 138. This causes the energization of indicating light 118 when the normal power supply across lines 28 and 30 is again re-est'ablished to thereby indicate that there has been an outage or failure of the power supply across lines 28 and 30.

As a further feature of the system, a reset timing device 140 is connected across the output terminals 94 and 96 of inverter 88 to time the interval of the auxiliary operation. Timing. device 140 may be reset to a zero reading at the end of the auxiliary operation. The output terminals 94 and 96 of inverter 88 across which the timing device 140 is connected are energized whenever relay 32 is de-energized by a failure of the normal power source across lines 28 and 30.

Second auxiliary power supply A second auxiliary supply in the form of an inverter 150 is provided for correcting negative time indications of clocks 10 and 12. Inverter 150 is connected to the storage battery 74 and to primary winding 22 of transformer 18 under the control of tandem-operated selector switches 34 and 64, respectively, and is not subject to control by relay 32. Inverter 150 is adapted to receive a 24-volt direct-current input from battery 74 at the input terminals 152 and 154 of the inverter, and is adapted to supply 115-volt, 90-cycle alternating-current power from its outputjterminals 156 and 158. Inverter 150 may be of the vibrator type.

Input terminal 152 of inverter 150 is connected by conductor 160-to fixed contact 66 of selector switch 64. Movable contact 72 of the selector switch is connected to conductor 110 and thus to terminal 75 of battery 74. Whenselector switch 64 is moved to the position where movable contact 72 engages fixed contact 66 of the switch, input terminal 152 of inverter 150 is connected to terminal 75 of battery 74 through conductor 160, fixed contact 66, movable contact 72, and conductor 110. The other input terminal 154 of inverter 150 is directly connected byconductor 104 to terminal 77 of battery 74.

The output terminal 156 of inverter 150 is connected by conductor 162 to fixed contact 52 of selector switch 34. When switch 34 is turned to the position in which movable contact 56 engages fixed contact 52, output terminal 156 of inverter 150 is connected through conductor 162, fixed contact 52, movable contact 56, and conductor 26 to terminal 23 of transformer primary 22. The other output terminal 158 of inverter 1501s connected by conductor 164, junction 58, and conductor 24 to terminal 25 of transformer primary 22. Hence, when the tandem-operated selector switches 34 and 64 are turned to the position in which their respective movable contacts 56 and 72 engage the respective fixed contacts 52 and 66, the input terminals 152 and 154 of the inverter are connected directly across battery 74 and the output terminals 156 and 158 are connected directly across the terminals 23 and 25 of transformer primary winding 22. When the tandem-operated selector switches 34 and 64 are in this position, clocks 10 and 12 are supplied with 115-volt, -cycle altemating-current power from inverter 150.

The movement of selector switch 34 away from the normal position in which movable contact 56 engages fixed contact 50, causes conductor 48 and movable contact 38 of relay 32 to be disconnected from terminal 23 of primary winding 22. This is effective to disconnect the normal power supply from primary winding 22, assuming movable contact 38 to be in engagement with fixed contact 40, or to disconnect the 60-cycle inverter 88 from transformer winding 22, assuming movablecontact 38 to be in engagement with fixed contact 42. Thus, it will be seen that the 90-cycle inverter 150 may be connected to clocks 10 and '12 by proper movement of selector switches 34 and 64, independently of whether the normal power supply is energizing power lines 28 and 30, or whether the normal power supply has failed and the inverter 88 is connected by relay 32. to selector switch 34. Since the normal power supply and the auxiliary power supply from inverter 88 are both fed through conductor 48Ito fixed contact 50 of selector switch 34, movement of contact 56 out of engagement with fixed contact 50 is effective to disconnect either the normal power supply from power lines. 28 and 30 or the output of inverter 88 from. the clocks, depending upon which of these power'supplies is connected through relay contact 38 to the selector switch contact 50.

The selector switches 34 and 64 also permit correction of positive time indication errors of clocks 10 and 12. When the respective tandem-operated switches are moved to the position in which movable contact-56 or switch 34 engages blank fixed contact 54, all sources of power are disconnected from primary winding 22. After the clocks have been disconnected from all sources of power for a sufficient time interval to correct the positive time indica: tion error of the clocks, selector switches 34 and 64 may again be moved to the normal position in which movable contact 56 of switch 34 engages fixed contact 50, and in which movable contact 72 of switch 64 engages fixed contact 68.

Summary of operation For normal operation, the tandem-operated selector switches 34 and 64 are placed in the position in which movable contact 56 of switch 34 engages fixed contact 50 of that switch, and in which movable contact 72 of switch 64 engages the blank fixed contact 68 of that switch.

If the normal a1ternating-current power supply, which is assumed to be volts, 60 cycles, is functioning to maintain lines 28 and 30 energized sufficiently to operate clocks '10 and 12, operating coil 36 of relay 32 is ener gized and moves movable contact '38 into engagement with fixed contact 40 and also moves movable contact44 out of engagement with fixed contact 46. The engagement of movable contact 38 with fixed contact 40 connects normal supply power line 28 to terminal 23 of primary winding 22 of transformer 18 through conductor 48, fixed and movable contacts 50 and 56 of selector switch 34 and conductor 26. The other terminal 25 of primary winding 22 is permanently connected through conductors 24 and 60 to power line 30. Hence, clocks 10 and 12 are operated directly from the normal alternating current power supply, the synchronous motors of the clocks operating in synchronism with the normal power supply from lines 28 and 30. Indicator light 116 which is connected directly across power lines 28 and 30 is lighted to indicate that the normal power supply is energizing power lines 28 and 30.

As long as power lines 28 and 30 remain energized, storage battery 74 is trickle-charged from rectifier 76 which is connected across power lines 28 and 30 through step-down transformer 80. A voltage of 24 volts is maintained across storage battery 74.

If the normal power supply across power lines 28 and 30 should fail, or if the voltage should drop below the value necessary to maintain clocks 10 and 12 in operation, relay 32 becomes de-energized, causing movable contact 38 to engage fixed contact 42 and causing movable contact 44 to engage fixed contact 46. The engagement of movable contact 44 with fixed contact 46 connects terminal 75 of storage battery 74 to input terminal 90 of inverter 88. The other terminal 92 of inverter 88 is already connected by means of conductors 100 and 104 to the other terminal 77 of storage battery 74. The engagement of movable contact 38 with fixed contact 42 connects. the l15-volt, 60-cycle alternating-current output from terminals 94 and 96 of inverter 88 across trans- [former primary winding 22. Output terminal 96 of the inverter is already directly connected by conductors 112, 60 and 24 to terminal 25 of primary winding 22. The other output terminal 94 of inverter 88 is connected through conductor 116, fixed contact 42 of relay 32, movable contact 38, conductor 48, fixed contact 50 of selector switch 34, movable contact 56 of selector switch 34, and conductor 26 to terminal 23 of primary winding 22. The 1l5-volt, 60-cycle output of inverter 88 is thereby directly connected to primary winding 22 when relay 32 is deenergized and selector switch 34 is in the normal position shown in the drawing. Clocks 10 and 12 are thereby energized with the ll-volt, 60-cycle output of inverter 88 and operate in synchronism with the output of the in verter.

Ifit should be noted that the clocks '10 and 12 have a negative error in their time indication, the tandem-operated selector switches 34 and 64 may be moved to the positions in which movable contact 56 of switch 34 engages fixed contact 52 and movable contact 72 of switch 64 engages fixed contact 66. With the selector switches 34 and 64 in the position just described, terminal 75 of storage battery 74 is connected through conductor 110, movable contact 72, fixed contact 66 and conductor 160 to input terminal 152 of 90-cycle inverter 150. Input terminal 154 of inverter 150 is already connected to the other terminal 77 of storage battery 74 through conductor 104. Thus, with movable contact 72 of selector switch 64 in engagement with fixed contact 66, input terminals 152 and 154 of inverter 150 are directly connected across the output of the storage battery.

Movement of movable contact 56 of selector switch 34 into engagement with fixed contact 52 connects output terminal 156 of inverter 150 through conductor 26 to terminal'23 of transformer primary winding 22. The other output terminal 158 of inverter 150 is already connected directly through conductors 164 and 24 to the other terminal 25 of transformer primary winding 22.

. Thus, when selector switches 34 and 64 are moved to the position in which movable contact 56 engages fixed contact 52 and movable contact 72 engages fixed contact 66, the 1'15-volt, 90-cycle output of inverter 150' is directly connected to primary winding 22 of transformer 18. With the 90-cycle output of inverter 150 connected to transformer 18 and thus to clocks and 12, the clocks are supplied with an input power of oneand one-half ti'mesthe normal firequency to thereby speed up theclocks to compensate for the negative error in the time indication of the clocks. When the negative error has been compensated for, selector switches 34 and 64 may be again returned to their normal position corresponding to the view shown in the drawing in which movable contact 56 engages fixed contact 50 on selector switch 34 and move ble cont-act 72 engages blank fixed contact 68 of selector switch 64.

If clocks 10 and 12 have a positive time error, this error may be compensated for by moving selector switches 34 and 64 to the position in which movable contact 56 of switch 34 engages blank fixed contact 54 of that switch and movable contact 72 of switch 64 engages blank fixed contact 70. When the switches 34 and 64 are moved to the positions just described, transformer primary winding 22 is disconnected from both the normal and auxiliary sources of power. Afiter the positive time error has been compensated for by disconnecting clocks 10 and 12 from all sources of power for a sufficient time to compensate for the positive error, switches 34 and 64 are again moved to their normal position in which the clocks are again connected to the 60-cycle output of either the normal power source or of inverter 88, depending upon the energization condition of relay '32.

If a power failure occurs, operating coil 122 of relay 1 20 associated with indicator light 11 8 is de-energized and permits contact '124 to move to a position in which it completes the circuit of indicator light 118 across power lines 28 and 30. Hence, when the normal power supply returns to power lines 28 and 30, indicator light 118 is lighted to thereby indicate that an outage or power failure has occurred of the normal power supply. Indicator light 118 may be extinguished by momentarily closing push button to thereby re-energize operating coil 122 of the relay. This causes movable contact 124 to move out of engagement with fixed contact 138 to thereby open the circuit of indicator light 118. Contact 124 provides a holding circuit for operating coil 122 when it engages fixed contact 130.

The timing means 140 connected across terminals 94 and 96 of inverter 88 is also energized as soon as failure of the normal power supply occurs, and times the interval of the outage of the normal power supply. Timing means 140 may be reset to its zero reading at the termination of the power outage in readiness for the next failure of the normal power supply.

Upon restoration of the normal power supply across power lines 28 and 30, operating coil 36 of relay 32 is again re-energized to thereby move movable contact 38 out of engagement with fixed contact 40 and to move movable contact 44 out of engagement with fixed contact 46. This results in disconnecting storage battery 74 from the input of inverter 88 and also disconnects the output of inverter 88 from selector switch =34 and thus from primary winding 22.

It should be noted that the selector switches 34 and 64 may be used to provide positive and negative corrections of the time indications of clocks 10 and 12 independently of whether or not the normal power supply across lines 10 and 12 has failed. Thus, assume that the normal power supply is properly energizing power lines 28 and 30 and it should be noted that clocks 10 and 12 have a positive error. :In such case, movement of contact 56 of switch 34 into engagement with blank contact 54 of that switch disconnects the normal power supply from transformer primary 22, thereby causing the clocks to stop. As soon as a suflicient time has elapsed to correct the posi tive error, switch 34 may again be thrown into the normal position in which movable contact 56 engages fixed con tact 50 to thereby reconnect the normal power supply across primary winding 22.

Similarly, assume that the normal power supply is properly energizing power lines 28 and 30, but it should be noted that clocks 10 and 12 have a negative time error, In such case, contacts 56 and 72 of selector switches 34 and 64 may be respectively moved into engagement with fixed contacts 52 and 66.

Movement of movable contact 56 into engagement'with fixed contact 52 and movement of movable contact 72 into engagement with fixed contact 66 connects the storage battery 74 to the input terminals ofinverter 150, disconnects the normal voltage supply of power lines 28 and 30 from the transformer primary winding 22, and connects the 115-volt, 90-cycle output of inverter 15010 primary winding 22. Thus, the'high-frequency output of inverter 150 is connected to clocks and 12 to compensate for the negative error. As soon as the negative error has been sufliciently compensated, switches Hand 64 may be returned to their normal position shown in the drawing, in which event the normal power supply from lines 28 and 30 is again connected to the clocks.

There is shown in FIGS. 2, 3 and 4 a modified apparatus and method which may be used for correcting negative time indicationsofthe clock system. In this modie m t q h h rm ncvrnv r s ch as a e ermanmtmasnet er i nected to the clock system and the frequency and voltage of the auxiliary power supply are increased at a predetermined rate not exceedinga maximum critical rate dependentzon the ability of the clock. system to stay in synchronism until a desired over-frequency is reached which is much higher than the normal supply frequency to the clock system.

One apparatus which may be used to supply this overfrequency power to the clock system may include a highfrequency alternator generally indicated at 170 having stator windings 172 and a rotor 174 which may be of the permanent magnet type. The magnets of the rotor may be of the type sold under the Aln-ico trademark, made of a highly magnetized alloy of aluminum, nickel and cobalt. The output leads 174 and 176 of alternator 170 are connected to the clock system through a suitable switch device.

Rotor 174 is driven by a capacitor motor generally indicated at 178 which is connected through power lines 180 and 182 to a suitable single-phase 60-cycle alternating current power supply. For example, if the power source connected to power lines 28 and 30 (FIG. 1) is in operation, capacitor motor 178 may be connected through a suitable switch means to power lines 28 and 30. On the other hand, if the normal power supply connected to power lines 28 and 30 has failed, motor 178 may be connected through a suitable switch device to the 60-cycle output of inverter 88, rather than having the clock system connected to the output of inverter 88, as in the embodiment of FIG. 1.

Capacitor motor 178 is of the single-phase type and includes two windings 184 and 186. One of the windings, such as winding 186, has a capacitor *188 connected in series with it to provide a phase displacement of the current in winding 186 with respect to the current in winding 184. A resistance 190 is also connected in series with winding 186 and capacitor 188 in order to limit the acceleration of motor 178 on starting. A flywheel 192 is also mechanically driven by capacitor motor 17 8 to limit acceleration of the motor.

The number of pairs of poles of rotor 174 of alternator 170 is selected to give a predetermined output frequency at the rated speed of motor 178, as indicated by the fiat horizontal line A on FIG. 3. This frequency is substantially higher than the normal frequency applied to the clock system indicated by the dotted line B in FIG. 3. For example, frequency A may be three or more times as great as normal frequency B.

With the output of alternator 170 connected to the clock system, motor 178 is connected to the 60-cycle power source and begins to accelerate so that the output frequency and voltage of alternator 170 increase along a curve indicated at C in FIG. 3 until the voltage and frequency of alternator 170 finally reach the level of line A which corresponds to the voltage and frequency of alternator -170 at the rated speed of motor 178. The rate of increase of frequency and voltage applied to the clock system should not exceed, but may be less than, the predetermined maximum rate indicated by the slope of the dotted line D'in FIG. '3. This maximum critical rate of frequency increase depends upon the physical characteristics of the synchronous clocks 10 and their ability to keep in step with the supply frequency as the frequency is being increased. I have found in practice that this maximum rate is approximately percent increase per second based on the original rated frequency of the clocks. In other words, if the rated frequency of the clocks is 60 cycles per second, then the rate of increase should not be greater than 60 cycles per second per second.

The rate of acceleration of motor 178 and consequently the rate of increaseof the voltage and frequency of alternator .may be suitably controlled by adjustment and proper proportioning of the flywheel 192 and also of the resistance in series with capacitor motor wind ing 186.

It can be seen from the foregoing that there is provided in accordance with this invention a master clock system inwhich the clocks'normally operate in; synchronism with the local power supply, but including a batterypowered auxiliary generating apparatus of the same frequency as the normal power supply which is automatically connected to the clocks in the event of the failure of the normal power supply. The provision of a second auxiliary power supply having a higher frequency than the normal power supply permits correction of negative errors in the time indications of the clocks. 'The selector switch permits the high-frequency auxiliary power supply to be connected to the clocks either with or Without a failure of the normal power supply having occurred, in order to correct for negative errors in the time indication of the clocks.

The system of the invention has the advantage of the inherent accuracy of a synchronous system over that of the conventional independent clock system which utilizes a master clock. At the same time, the present system has the synchronization control advantages that are possible with an independent clock system and also avoids the cumulative errors inherent in the usual pendulumcontrolled master clock systems. The system of the present invention is also considerably more economical to install than the conventional independent clock system. The system has the further advantage that it operates on a low-voltage power supply, thereby permitting the clocks to be connected to power through low-voltage conductors, such as twisted telephone wires, further adding to the simplicity and economy of the system.

The modified method and apparatus for correcting negative time errors of a clock system in which the frequency applied to the clocks is increased greatly over the normal supply frequency while maintaining the rate of increase within a critical limiting rate, also has many advantages. This method permits the negative time error of the clock system to be corrected very rapidly since the applied frequency to the clocks may be made much higher than the normal rated frequency of the clocks.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and, therefore, it is aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as my invention is:

1. In an electric clock system including an electric clock driven by an alternating current motor, and a normal source of alternating current power of a given frequency normally connected to said alternating current motor; a correction device comprising an energizable drive motor, an alternator coupled to said drive motor and drivable by said drive motor when energized to generate alternating current of a frequency proportional to the speed of said drive motor, said drive motor having a normal operating speed at which said alternator generates current of a frequency at least twice said given frequency, means for disconnecting said alternating current motor from said normal source and simultaneously connecting said alternating current motor to said alternator and energizing said drive motor, and means for retarding the acceleration-of said drive motor to prevent said alternator from achieving a frequency equal to said normal freqiiencywithin the first second after energization of said drive motor. I

=2. In 'an electric clock system including an electric clock driven by an alternating current motor, and a normal source of alternating current power of a given frequency normally connected to saidialternating current motor; a correction device comprising an cnergizable drive motor, an alternator coupled to said drive motor and drivable by said drive motor when energized to generate alternating current of a frequency proportional to the speed of's'aid drive motor, said drive motor having a normal operating speed at which said alternator generates current of a frequency several times said given frequency, means for disconnecting said alternating current motor from said normal source and simultaneously connecting said alternating current motor to said alternator and energizing said drive motor, and means for regulating said motor to maintain the magnitude of the rate of increase of frequency of said alternator below the magnitude of said given frequency.

References Cited in the file of this patent UNITED STATES PATENTS 1,476,978 Jeifrey Dec. 11, 1923 1,526,539 Fraser Feb. 17, 1925 1,533,906 Threm Apr. 14, 1925 1,598,268 Coninck Aug. 31, 1926 1,767,949 Warren June 24, 1930 2,377,617 Dicke et a1. June 5, 1945 FOREIGN PATENTS 658,004 Germany Mar. 19, 1938 614,751 Great Britain Dec. 22, 1948 

